> Why would I buy anything with my bitcoins if a week from now they're going to be work X% more? I'd just wait until then.

If you believe this, then surely you also believe you should invest 100% of your money in bitcoins?

If not, then you believe only X% of your money should be invested in bitcoins (to avoid the risk of losing everything in a collapse)

Therefore if you have >X% of your money in bitcoins, you should spend some.